Open system fluid driven pruner

ABSTRACT

An open system fluid driven pruner, which can readily be converted for use as a closed system pruner, wherein only that magnitude of pressure required to cut a given tree limb or the like is generated, and whereby, once the work stroke has been completed, no pressure is exerted upon the pruner. The preferred pruner is reversible, so that displacement of the cutting blade may occur in either direction, depending upon whether a given embodiment is a standard pruner or a hook pruner, and comprises a valve housing for applying fluid under pressure against a piston, displacement of which pivots the pruner cutting blade against an anvil, the blade and anvil being carried upon an extension which allows cutting of hard to reach limbs. Once the piston has completed the work stroke it functions as a valve, releasing blade-actuating pressure.

United States Patent Lind Dec. 2, 1975 1 1 OPEN SYSTEM FLUID DRIVEN PRUNER Primary ExaminerA1 Lawrence Smith Assistant Examiner,l. C. Peters Attorney, Agent, or FirmLynn G. Foster [75] Inventor: Michael A. Lind, Salt Lake City,

Utah [57] ABSTRACT [73] Ass1gnee: New Draulics, Inc., Salt Lake City,

Utah An open system fluid driven pruner, which can readily be converted for use as a closed system pruner, [22] Filed: Nov. 29, 1974 Appl. No.: 528,071

wherein only that magnitude of pressure required to cut a given tree limb or the like is generated, and whereby, once the work stroke has been completed, no pressure is exerted upon the pruner. The preferred pruner is reversible, so that displacement of the cutting blade may occur in either direction, depending upon whether a given embodiment is a standard pruner or a hook pruner, and comprises a valve housing for applying fluid under pressure against a piston, displacement of which pivots the pruner cutting blade against an anvil, the blade and anvil being carried upon an extension which allows cutting of hard to reach limbs. Once the piston has completed the work stroke it functions as a valve, releasing blade-actuating pressure.

1 Claim, 3 Drawing Figures 52] U.S. Cl 30/228; 91/401 [5 l] Int. Cl. B26B 15/00 [58] Field of Search 30/228, 180, 272 R; 91/401 [56] References Cited UNITED STATES PATENTS 1,744,069 1/1930 Dapron 91/401 X 2,543,109 2/1951 Ho1owka.. 30/228 2,815,004 12/1957 Droman... 30/228 X 3,476,015 11/1969 Ackleymn 91/401 X 3,654,834 4/1972 Sifri 91/401 FLUID 22 re 5564 ll 34 24 52 Z M382 I20 I06 92 I: 1. 08 (no I32 I 4' 1%,, am i U.S. Patent Dec. 2, 1975 Sheet 2 of 2 3,922,782

OPEN SYSTEM FLUID DRIVEN PRUNER BACKGROUND 1. Field of Invention The present invention relates generally to the field of cutting instruments, and more particularly to a novel pruner for cutting limbs from trees and the like.

2. Prior Art Prior art pruners known to the applicant comprise closed hydraulic systems wherein a spring controlled relief valve circulates hydraulic fluid in such a fashion that the pruner becomes hot, making the pruner uncomfortable to handle and inherently inefficient.

Also, the pressure level existing during cutting is often far in excess of that needed, reducing the useful life of the pruner and increasing maintenance costs. These problems are accentuated because, once the cutting stroke is complete, the actuating components of the pruner remain under the mentioned high pressure.

In addition, most prior art purners have not been adaptable for use in both standard and hook pruner configuration.

BRIEF SUMMARY AND OBJECTS OF THE PRESENT INVENTION In summary, pruners according to the present invention comprise an open fluid system wherein the tool does not heat to uncomfortable temperatures, only sufficient pressure to complete a cutting function is generated, and pressure is released from the actuating parts of the tool during non-use as well as immediately following each cutting stroke. The preferred tools are reversible for standard as well as hook pruner utilizations.

It is a primary object of the present invention to provide a novel pruner and related methods.

It is also a principal object of the present invention to provide an improved pruner which is reversible for utilization in standard as well as hook pruner configurations.

It is a further important object of the present invention to provide a novel pruner and method whereby pressure is applied to the actuating components only during each cutting stroke.

It is a paramount object of the present invention to provide a novel pruner and method whereby only that pressure needed to drive the actuating components through the cutting stroke is generated.

These and other objects and features of the present invention will be apparent from the following detailed description, taken with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, comprising FIG. la and lb, is a longitudinal cross section of a presently preferred embodiment of this invention shown in a standard pruner configuration; and

FIG. 2 is a fragmentary longitudinal section of another presently preferred embodiment of this invention shown in a hook pruner configuration.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS With reference to FIG. 1, a standard pruner is illustrated which comprises a valve assembly 12, a piston-cylinder portion 14, an extension section 16 and a cutting portion 18.

The valve assembly 12 comprises a valve housing 20 in which a valve 22 is reciprocably disposed essentially transverse of an influent port 24 and an effluent port 26. A conventional hydraulic system is threaded to the interior of ports 24 and 26 to provide and exhaust hydraulic fluid under pressure in a conventional manner. Each of the ports 24 and 26 communicates via channels 28 and 30, respectively, to the valve bore 32.

The valve comprises reduced diameter rod ends 34 and 36 respectively connected to opposed ends of a central spool 38 of the valve 22. The ends 34 and 36 of the valve 22 are exposed so that, by pushing upon end 34, the handle 40 may be actuated where the handle per se is difficult to reach, and the end 36 is secured to the handle 40 by roll pin 42. The handle 40 is centrally joined at pin 44 to lug 45 of housing 20 by toggle 47, which is also comprised of a handle-engaging pin 49. Thus, counterclockwise rotation of the handle 40 about pivot pin 44 results in a displacement of the spool 38 from the solid to the dotted position shown in FIG. 1. The purpose of such displacement will hereinafter be more fully described.

Release of the handle 40, following actuation, results in a restoration of the valve 22 to the solid line position of FIG. 1 by reason of the force exerted against the spool 38 by compression spring 46. Not only does the compression spring 46 abut one end of the spool 38, but also abuts the interior surface 48 of a threaded plug 50, which is threadedly received into the valve housing 20 and provides a sealed condition with the housing 20 and the end 36 of the valve 22 at O-rings 52 and 54, respectively. An identical plug is interposed between the end 34 of the valve 22 and the valve housing 20. Thus, the valve 22 may be manually reciprocated between the solid and dotted line positions of FIG. 1 without resulting fluid leakage from the housing 20.

The valve bore 32 at its central most point communicates with a passage 60, while the bore 32 communicates with a passage 62 adjacent valve end 34 and a passage 64 adjacent valve end 36, for purposes later to be described.

The piston-cylinder section 14 comprises a cylinder in the form of a rigid sleeve the exterior surface at each end 82 and 84 being threaded and the exterior surface of the cylinder 80 between the threads being circumscribed by a handle cover 86 of suitable antifriction material. Threads 82 are threadedly secured to an opening 88 in the valve housing 20, an O-ring 90 sealing the threaded connection against fluid leakage.

An annular piston assembly 92 reciprocates along the inside wall of the cylinder 80 in a manner and for purposes hereinafter more fully explained. Said reciprocation is also in respect to a stationary center cylindrical tube 94, the left end of which is telescopically received by threaded sleeve 96 adjacent the passage 60 of the valve housing 20, being sealed in that position by O- ring 98 and secured by tube nut 100, which is integral with the sleeve 96. Fluid under pressure in passage will thus be displaced through the hollow interior 102 of the tube 94.

It is to be appreciated that the passageways 62 and 64 communicate directly with the hollow interior 104 of the cylinder 80 external of the central cylindrical tube 94. Thus, pressure is exerted upon the left side of the piston 92 as viewed in FIG. 1.

The piston assembly 92 comprises an annular peripheral element 106 having an annular groove 108 in which a T-seal 110 is disposed such that the annulus 106 fits snugly though slidably against the interior of the cylinder and the T-seal 110 prevents fluid transfer between the exterior surface of the annulus 106 and the internal surface of the cylinder 80.

The annulus 106 comprises a plurality of longitudinally directed through bores 112 for selective fluid communication in a manner and for purposes hereinafter explained.

The hollow interior of the annulus 106 is threaded at 116 and stepped at 118 to snugly, though slidably engage the exterior surface of the tube 94 for piston reciprocation, T-seal 120 preventing passage of fluid under pressure across the interior of the annulus 106.

A partially hollow piston rod 122, having side ports 124, accommodates reverse of the fluid flow emanating from the hollow interior 102 of the tube 94 back along the right side of the piston assembly 92 within the hol' low 104 of the cylinder 80. The piston rod 122 extends to the right of the piston-cylinder assembly 14 in a manner and for purposes hereinafter explained.

A plurality of rods are slidably disposed in longitudinal bores 132, the ends of which are secured to left and right rings 134 and 136 respectively by countersunk screws, so that the rings 134 and 136 may be jointly displaced in respect to the annulus 106 left or right to close the bores 112 in a selective manner later to be described.

It is to be appreciated that the opening 88 at the right of the valve housing 20 is stepped at 140 and defines a shoulder 141 to allow the right 134 to bottom out on the shoulder as the annulus 106 engages the shoulder 143, the distance between the shoulders 141 and 143 insuring displacement of the rings from a closed to an open position.

The right end of the interior 104 of the cylinder 80 adjacent the piston rod 122 is closed by a cylindrical plug 150, which is flanged at 152 thereby covering the right end of the cylinder 80. The free right end of the tube 94 terminates near the plug 150, while the piston rod 122 extends through the hollow center of the plug in contiguous relation with a reduced diameter portion 154, which comprises a groove receiving a T- seal 156. The plug 150 comprises a stepped bore 158 defining a shoulder 159, which respectively loosely receives the ring 136 until the ring bottoms out on the shoulder shifting the piston assembly from a closed to an open position, as the annulus 106 engages the shoulder 161. The exterior surface 160 of the plug 150 comprises a groove which receives an annular O-ring 162 for fluid sealing purposes.

The threads 84 of the cylinder 80 are threadedly engaged by interior thread of a cylindrical end cap 164, which is externally tapered and internally stepped. The end cap 164 retains a thrust bearing 165 adjacent the right end of the plug 150 as well as a flange bearing 168. The flange bearing is retained by being press fit into the end cap 164. A ring 167 is interposed between the thrust bearing 165 and the flange bearing 168, being threadedly secured at 169 to the left end of an outer extension tube 172. A set screw 170 insures unitary rotation of the ring 167 and outer extension tube 172 in relation to the cylinder 80, whereby the cutting portion 18 is rotated to facilitate the pruning of limbs having varied orientations.

The piston rod 122 is coaxially within the extension tube 172. The piston rod 122 is stepped at 174, 176 and 178, respectively. A key 180 connects the mentioned stepped portion 176 with a push rod 182, which comprises an extension of the piston rod 122. Thus, with each stroke left to right of the piston assembly 92, the push rod 182 displaces to the right within the outer extension tube 172, with the opposite occurring during right to left movement of the piston assembly 92. The outer extension tube 172 and push rod 182 may be of any desired length, depending upon the anticipated relative positions of limbs to be pruned and the operator of the pruner, and swivel together about reduced diameter portion 174.

The cutting portion 18 comprises an exterior sleeve 184, which is threadedly or otherwise engaged to the distal end of the outer extension tube 172 so that each are longitudinally stationary during use of the tool 10. A yoke clevis 186 is exteriorly threadedly connected to the distal end of the push rod 182 in such a way that the yoke clevis 186 and the push rod 182 unitarily reciprocate within the sleeve 184 and tube 172. A dowel pin 188 pivotally joins a yoke arm 190 to the yoke clevis 186 at its slot 192 such that reciprocation of the yoke clevis 186 likewise reciprocates the yoke arm 190, the arm 190 projecting eccentrically through the outer extension tube 172 at slot 193 and through the sleeve 184 at elongated slot 194.

The anchor end 196 of an anvil 198 is secured by a roll pin 200 to the tube 172 and sleeve 184, the pin 200 passing transversely therethrough to both the sleeve 184 and the sleeve 172. A bolt lock ring 201 having a plurality of interior teeth 204 is provided at the main aperture thereof for locking a blade bolt 206 in a secured position, the threaded end of the bolt 206 receiving a nut (not shown). A locking clip 207 held by screws 202, further secures the blade bolt against inadvertent removal.

The blade bolt 206 passes through aligned apertures in the anvil 198 and an adjacent cutting blade 210, a lug 212 of the cutting blade 210 being secured by a pin 214 in the slot ofa blade clevis 216. The bolt 206 is further secured to the distal end of the tool by one of a plurality of machine screws 202 (only one of which is shown), held in place by suitable washers. The proximal interior threaded end 218 of the blade clevis 216 is threaded to the forward distal end of the yoke arm 190. Thus, left to right displacement of the piston, piston rod and yoke arm will cause the blade 210 to rotate clockwise about the blade bolt 206 bringing the cutting edge 211 of the blade through a limb being cut passed the anvil 198. It is to be appreciated that if the flow through characteristics of the piston assembly 92 are not desired, a solid piston may be used in lieu thereof, in which case the system becomes a closed system as opposed to an open system.

Reference is now made to FIG. 2 which illustrates in fragmentary side elevation, parts broken away for clarity, a second presently preferred embodiment of the present invention, i.e. a hook pruner 250. The hook pruner 250 comprises the valve assembly 12 (not shown in FIG. 2), the piston-cylinder assembly 14 (not shown in FIG. 2) and the extension section 16 only the right hand portion of which is shown in FIG. 2. Since the portions 12, 14 and 16 of the hook pruner 250 of FIG. 2 are substantially identical to components 12, 14 and 16 of the pruner 10 of FIG. 1, no additional description is believed necessary at this juncture.

The push rod 182 in the hook pruner 250 embodiment will be somewhat longer than the corresponding push rod of the pruner since the cutting stroke of the pruner 250 is right to left as opposed to left to right. Otherwise, the yoke clevis 186, the pin 188, the slotted outer extension tube 172, the slotted sleeve 184 and the yoke arm 190 are all substantially the same as described in conjunction with FIG 1.

The cutting portion 252 of the pruner 250 comprises an elongated blade clevis 254, which is threadedly joined to the distal end of yoke arm 190 and terminates well beyond the distal end of the sleeve 184 and the outer extension tube 182. A clevis pin 256, appropriately secured in position by a cotter pin or the like pivotally joins the distal end of a cutting blade 258 to the distal slotted end of the clevis 254. The blade 258 comprises a cutting edge 260.

One or more roll pins 262 secures the stud end 264 of a cantilevered anvil 266 in stationary relation to the sleeve 184 and outer extension tube 182. A blade bolt 268 pivotally secures the blade 258 near its center to the anvil beam 270. Thus, right to left displacement of the piston rod and push rod 182 will likewise displace from right to left the yoke arm 190 and the blade clevis 254 forcing the blade 258 to rotate about the blade bolt 268 causing the cutting edge 260 to cross the anvil 260 thereby cutting a limb interposed between the blade and the anvil.

In use, the standard pruner of the FIG. 1 is attached to a suitable source of hydraulic fluid under pressure at ports 24 and 26 of the valve housing 20. So long as the handle 40 is in the position illustrated in solid lines in FIG. 1, hydraulic flow will be as indicated by arrows 220, ultimately resulting in a closing of the ports 112 by the right disc 136, following which the piston assembly 92 moves to its leftmost position and thereafter functions as a valve opening once more the bores 112 upon engagement of the disc or ring 134 with the shoulder 141 adjacent the step 140 of the bore or opening 88 of the valve housing 20. Thereafter, fluid is readily evacuated through port 26 along a path indicated by arrows 222.

When the handle 40 is displaced moving the spool 38 to the dotted position shown in FIG. 1, the spool 38 connects the bore 60 with the port 26 and influent fluid entering port 24 passes along the passageway 28, across the valve bore 32, along the passage 62 and impinges upon the left side of the piston assembly 92. This pressure causes the ring 134 to close the bores 112 of the piston assembly 92 and the fluid pressure drives the piston assembly 92 from left to right thereby displacing the piston rod 122 together with the push rod 182 and the yoke clevis 186 to the right forcing the cutting edge 211 of the blade 210 clockwise about bolt 206 through a limb being cut passed the anvil 198. A pressure build up against the left side of the piston assemblage 92 is restricted to that level of pressure required to force the cutting edge 211 of the blade 210 through the limb. Once the blade 210 has completed its stroke, the piston assembladge 92 will be situated with the ring 136 entering the step 158 and bottoming out against the adjacent shoulder 159 causing the bores 112 to once more open and relieve pressure against the piston. Thus, with the piston assembly 92 in its forward extreme position and vented, and the valve 22 in the dotted position, influent fluid will be vented through the piston bore 112, the

ports 124 of the piston rod 122, the tube 94, the bore 60 and out the exhaust port 26.

Upon release of the handle 40, the compression spring 46 returns the valve 22 to its initial solid line position as shown in FIG. 1, following which influent fluid received at port 24 follows the path designated 220 impinging upon the disc 136, causing the disc to close the bores 112 following which the piston assembly 92 moves from the right-most position to the leftmost position in the cylinder. Thereupon the bores 112 are once more opened and fluid is exhausted along flow path 222 to the effluent port 26.

It is to be appreciated that the hydraulic hook up of the pruner 250 is opposite to the hydraulic hook up of the pruner 10, Le. influent fluid is received at port 26 and effluent fluid exhausted at port 24. Thus, actuation of the handle 40 of valve 22 will displace the piston assembly 92 from a rightmost position to a leftmost position causing a corresponding right to left displacement of the yoke arm and the blade clevis 254 resulting in rotation of the cutting blade 258 about the blade bolt 268 bringing the cutting edge 260 of the blade 258 through a limb and passed the anvil 266.

It is to be appreciated that the materials used in fabricating various components of pruners 10 and 250 may be of any suitble type. Aluminum is presently preferred, although when it is expected that pruners according to the present invention will be used adjacent power lines, part or all of the extension section 16 and other parts, if desired, may be of F iberglas or other dielectric material to electrically insulate the user from the cutting portion.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. A pruner mechanism comprising:

elongated support means;

anvil means carried by and exposed at the distal end of the elongated support means;

elongated reciprocable drive means substantially coextensive with said elongated support means;

cutting blade means carried by and exposed at the distal end of the elongated reciprocable drive means, the cutting blade means being pivotable in respect to the anvil means whereby a cutting edge of the blade means engages the anvil means upon displacement of the drive means in one direction, the proximal end of the elongated reciprocable drive means being hollow for a length greater than the distance through which said reciprocation occurs and having port means therethrough; cylinder-piston assembly comprising hollow cylinder means anchored to the proximal end of the elongated support means and further comprising annular piston means reciprocable within the cylinder means and anchored to and reciprocable with the hollow proximal end of the elongated drive means, the piston means being located rearwardly of the port means, displacement of the piston means causing a corresponding displacement of the drive means;

manually operable control valve means carried adjacent the cylinder-piston assembly and having displaceable means for selectively communicating fluid under pressure along two of three flow paths at any point in time to and from either side of said piston means within the cylinder means;

hollow conduit means spanning between the displaceable means of the manually operable control valve means and the interior of the hollow proximal end of the elongated reciprocable drive means, the hollow conduit means being concentric within said proximal end and communicating fluid in respect to said hollow proximal end;

first annular seal means being interposed between the interior of the annular piston means and the exterior of the hollow conduit means;

second annular seal means interposed between the interior of the cylinder means and the piston means;

first passageway means disposed between the displaceable means of the control valve means and the hollow interior of the cylinder means at the trailing end thereof;

second passageway means disposed between the displaceable means of the control valve means and the hollow interior of the cylinder means at the trailing end thereof;

whereby (a) when the displaceable means of the control valve means are located in a first position fluid under pressure is simultaneously though oppositely displaced between the piston means and said displaceable means through (1) one of said passageway means and (2) the hollow conduit means, and (b) when the displaceable means of the control valve means are located in a second position fluid under pressure is simultaneously though oppositely displaced between the piston means and said displaceable means through (3) the other of said passageway means and (4) the hollow conduit means;

said annular piston means carrying unbiased pressure responsive fluid valve means which reciprocate in respect to the annular piston means and then abut the ends of the cylinder means and thereby open at each extreme position along the path of reciprocation of the piston means between the ends of the cylinder means and close under force of fluid pressure exerting upon one side or the other of the piston means intermediate said extreme positions, the annular piston means comprising at least two bores through the annulus thereof defining front and back openings thereto each at opposite end faces of the piston means, one of said bores selectively communicating fluid across the piston means between a leading chamber of the cylinder means and a trailing chamber of the cylinder means, the bores being radially offset from the axial center line of the piston means about the same radial distance, the unbiased pressure responsive reciprocable fluid valve means comprising (a) at least one axially directed rod means reciprocably extending through one said bore and having ends exposed beyond the front and back openings thereof and (b) unbiased annular plate valve means respectively mounted to the opposed ends of the rod means in substantially parallel planes each substantially normal to the axes of the piston means and the two bores, the distance between the two unbiased annular plate valve means being substantially greater than the length of the two bores, the size, shape and dimensions of each unbiased annular plate valve means causing the plate valve means to cover and close the adjacent opening of the fluid communicating bore when said plate valve means is contiguous with the adjacent end of the piston means whereby the fluid communicating bore is always closed by one of the plate valve means during displacement of the piston means between the ends of the cylinder means and opened upon butting either end of the cylinder means. 

1. A pruner mechanism comprising: elongated support means; anvil means carried by and exposed at the distal end of the elongated support means; elongated reciprocable drive means substantially co-extensive with said elongated support means; cutting blade means carried by and exposed at the distal end of the elongated reciprocable drive means, the cutting blade means being pivotable in respect to the anvil means whereby a cutting edge of the blade means engages the anvil means upon displacement of the drive means in one direction, the proximal end of the elongated reciprocable drive means being hollow for a length greater than the distance through which said reciprocation occurs and having port means therethrough; a cylinder-piston assembly comprising hollow cylinder means anchored to the proximal end of the elongated support means and further comprising annular piston means reciprocable within the cylinder means and anchored to and reciprocable with the hollow proximal end of the elongated drive means, the piston means being located rearwardly of the port means, displacement of the piston means causing a corresponding displacement of the drive means; manually operable control valve means carried adjacent the cylinder-piston assembly and having displaceable means for selectively communicating fluid under pressure along two of three flow paths at any point in time to and from either side of said piston means within the cylinder means; hollow conduit means spanning between the displaceable means of the manually operable control valve means and the interior of the hollow proximal end of the elongated reciprocable drive means, the hollow conduit means being concentric within said proximal end and communicating fluid in respect to said hollow proximal end; first annular seal means being interposed between the interior of the annular piston means and the exterior of the hollow conduit means; second annular seal means interposed between the interior of the cylinder means and the piston means; first passageway means disposed between the displaceable means of the control valve means and the hollow interior of the cylinder means at the trailing end thereof; second passageway means disposed between the displaceable means of the control valve means and the hollow interior of the cylinder means at the trailing end thereof; whereby (a) when the displaceable means of the control valve means are located in a first position fluid under pressure is simultaneously though oppositely displaced between the piston means and said displaceable means through (1) one of said passageway means and (2) the hollow conduit means, and (b) when the displaceable means of the control valve means are located in a second position fluid under pressure is simultaneously though oppositely displaced between the piston means and said displaceable means through (3) the other of said passageway means and (4) the hollow conduit means; said annular piston means carrying unbiased pressure responsive fluid valve means which reciprocate in respect to the annular piston means and then abut the ends of the cylinder means and thereby open at each extreme position along the path of reciprocation of the piston means between the ends of the cylinder means and close under force of fluid pressure exerting upon one side or the other of the piston means intermediate said extreme positions, the annular piston means comprising at least two bores through the annulus thereof defining front and back openings thereto each at opposite end faces of the piston means, one of said bores selectively communicating fluid across the piston means between a leading chamber of the cylinder means and a trailing chamber of the cylinder means, the bores being radially offset from the axial center line of the piston means about the same radial distance, the unbiased pressure responsive reciprocable fluid valve means comprising (a) at least one axially directed rod means reciprocably extending through one said bore and having ends exposed beyond the front and back openings thereof and (b) unbiased annular plate valve means respectively mounted to the opposed ends of the rod means in substantially parallel planes each substantially normal to the axes of the piston means and the two bores, the distance between the two unbiased annular plate valve means being substantially greater than the length of the two bores, the size, shape and dimensions of each unbiased annular plate valve means causing the plate valve means to cover and close the adjacent opening of the fluid communicating bore when said plate valve means is contiguous with the adjacent end of the piston means whereby the fluid communicating bore is always closed by one of the plate valve means during displacement of the piston means between the ends of the cylinder means and opened upon butting either end of the cylinder means. 